THE FOOD OF PLANTS 129 
do not become food until a considerable amount of work 
has been done upon them by the plant itself. 
It thus appears that, in the strict sense, the ordinary 
green plant does not absorb its food from without. It 
takes in various raw materials from which it manufactures 
its food in particular parts of its own tissues. 
In connection with the nutrition of plants we have thus 
to deal with the absorption of the crude food materials, 
and to study the changes which they undergo after such 
absorption. But this is not all; the food which is manu- 
factured from them is not merely prepared in answer to 
the immediate requirements of the moment. A consider- 
able excess is usually constructed, and the surplus quantity 
is stored in various parts of the plant’s body for subsequent 
consumption. 
The food which is thus laid up in seeds, tubers, bulbs, 
&c. is not deposited there in exactly the condition in which 
the living substance requires it, so that there remains for 
us to consider the processes of storage, and the changes 
which the stored materials subsequently undergo for the 
purpose of feeding the living protoplasm. 
The construction of food from the materials absorbed is 
one of building up complex bodies from simple materials. 
The utilisation of the stored surplus is comparable with 
the digestion which is so marked a feature of animal alimenta- 
tion, and is one of breaking down of complex bodies into 
simpler ones. 
The actual nutrition of the protoplasm shows again two 
distinct phases : the incorporation into its substance of the 
ultimate constituents of the food, or its assimilation, is a 
constructive process ; it is in turn associated with a destruc- 
tive one, an auto-decomposition of the protoplasm itself, by 
which simpler bodies are produced from it. 
The whole round of changes which embraces all these 
operations is called metabolism, the constructive processes 
being grouped together under the name of anabolism, the 
destructive ones under that of katabolism. 
9 
